1. Field of the Invention
The present invention relates to a piezoelectric motor and a piezoelectric motor system driving a driven target using a piezoelectric unit.
2. Description of the Related Art
Conventionally, in the field of multi-degree-of-freedom rotary-drive control systems, typified by the orientation control of a surveillance camera, a joint of a robot, and the like, a drive mechanism or motor system is in wide use with a structure in which single-axis drive motors are stacked in series. Moreover, from the viewpoint of achieving miniaturization and higher precision, sometimes a multi-degree-of-freedom drive mechanism or multi-degree-of-freedom motor system is used that includes a support system mainly composed of a gimbal mechanism and a joint mechanism and a drive system mainly composed of a separately provided electromagnetic motor and the like. However, the conventional drive mechanism or motor system with the basic structure mainly composed of the gimbal mechanism and joint mechanism has not always been satisfactory because the structure is complicated and there is a limit to miniaturization. In light of such circumstances, studies and development of spherical motors using piezoelectric elements have been attracting attentions in recent years. Piezoelectric motors in particular, which drive a sphere (as a driven target) by frictional force using a piezoelectric unit, are regarded as promising for next-generation spherical motors that have a small size and high precision.
For a piezoelectric motor using a piezoelectric element, a device for changing the angle of a ball joint is known. In this device for changing the angle of a ball joint, three oscillators (piezoelectric units) are disposed in a predetermined arrangement inside a ball housing member that houses a ball member of the ball joint, and respective tip portions of these three oscillators abut on the ball. To maintain of the abutment, the ball is configured to be energized towards the oscillators by a plurality of spring members similarly disposed in a predetermined arrangement inside the ball housing member. In this state, each of the joined portions of these oscillators is rotated in a desired, predetermined direction, and thus the ball member of the ball joint is rotated in a desired direction. Thereby, the angle of the ball joint is changed in a predetermined direction. Each oscillator (piezoelectric unit) is comprised of three oscillation elements (piezoelectric elements) that are integrated into a single body with their tip portions joined to each other, and the oscillation elements (piezoelectric elements) are disposed in positions corresponding to three edges of a regular triangular pyramid, with their center at the tip of the pyramid.
As another conventional example of a piezoelectric motor using a piezoelectric unit, an image pickup apparatus is known in which an image pickup unit is substantially spherical, with its circumferential surface formed to be convexly spherical, except for an image pickup optical system portion. Additionally, an image pickup optical system and an image pickup device, such as a CCD, are provided inside the image pickup unit. A casing (spherical) of the image pickup unit is formed of for example, magnetic material, and the convexly spherical surface portion thereof is attracted to a holding member in the form of an annular magnet. The holding member is fixed to the main body of the image pickup apparatus. Consequently, the image pickup unit is held on the main body of the image pickup apparatus, in a moveable manner, by the magnetic force of the holding member. A drive mechanism is provided at the center of a central aperture portion of the annular holding member. The drive mechanism includes a two-dimensional piezoelectric actuator, a frictional member provided at an approximately central portion of the actuator, and the like. The frictional member is configured to abut the convexly spherical surface of the image pickup unit and to move along with the extension and contraction of each arm of the two-dimensional piezoelectric actuator so as to allow the image pickup unit to move in a predetermined manner.
According to the above-described device for changing the angle of a ball joint and image pickup apparatus, the complexity of the gimbal mechanism and joint mechanism can be overcome, and a small-sized, multi-degree-of-freedom drive mechanism can be achieved.
Since a piezoelectric motor of this type is driven by friction, the piezoelectric motor needs to be used in such a manner that the piezoelectric unit is pressed against the driven target. However, the above-described device for changing the angle of a ball joint has a structure in which a preload force is applied by using the plurality of spring members. This structure, in practice, requires a preload part composed of the spring members and, on the periphery of the preload part, a guide part for rotatably supporting the ball member. The preload part and guide part have led to upsizing of the motor, as well as a reduction in the range of movement and an increase in the drive load (an increase in idle torque) due to friction. In addition, since the ball member, oscillators, and plurality of spring members are all disposed inside the ball housing member, the problem has arisen that the device as a whole is further increased in size.
Moreover, in the above-described image pickup apparatus, a guide part for rotatably supporting the image pickup unit is structured in such a manner that the holding member composed of the annular magnet attracts the convexly spherical surface portion of the image pickup unit. Accordingly, the drive load due to the friction at the guide part is considerably increased (a considerable increase in idle torque). For this reason, the drive mechanism needs to generate a driving force sufficient to overcome the drive load due to the friction. As a result, the problem has arisen that the apparatus as a whole is increased in size.